Raster scan photoplotters of both planar and internal drum design are known in the art. These devices are used in the fabrication of printed circuit boards. Planar photoplotters or imagers such as are disclosed and claimed in U.S. Pat. No. 4,851,656 and incorporated herein by reference have a planar surface for receiving a substrate. An optical exposure head is located on a movable gantry apparatus and is rastered above the substrate during exposure. Internal drum photoplotters have a cylindrical surface portion to receive the substrate. The exposure beam emanates from an optical exposure head and is scanned across the substrate. The optical exposure head is indexed along the longitudinal axis of the cylinder to complete the substrate exposure.
Internal drum raster photoplotters have inherent advantages over planar type scanners for several reasons, including simplicity of design and lower costs. However, both are subject to component tolerances which result in lower accuracy than would otherwise be possible. The drum surface is fabricated with inherent deviations from perfect cylindricity. Planar photoplotters or flatbed scanners are similarly afflicted by surface irregularities. These deviations result, in part, in scan lines (or segments thereof) on the substrate of varying lengths.
A compensation technique adaptable for use with a planar photoplotter or scanner is found in U.S. Pat. No. 3,555,254, incorporated herein by reference. Disclosed therein is a system for positioning a driven part in a numerically controlled positioning device. In the setting up of the system, the driven part is commanded to move to various positions spread over its field of movement and after it reaches each such position, its actual position is accurately measured to determine the error between the commanded position and the actual position. The values of the errors thus determined are stored in computer memory as a table of error values versus part position. Thereafter, as the driven part is moved to different positions relative to the reference member, the computer memory is interrogated and error values from the table are used to correct the commands transmitted to the motor drivers to take into account the repeatable error associated with the position of the driven part. The '254 system further includes an input device for providing position input commands, one or more motors for driving the driven part, and a computer for converting the input commands into corrected output commands.
In the prior art, internal drum raster photoplotters have been built with component defect compensation. For example, MDA of Vancouver, BC markets an internal laser raster drum photoplotter with compensation. First, the deviations from true cylindricity are determined as a function of position on the cylinder portion surface. The commanded raster pattern is thereafter shifted by incremental pixels at the photoplotting resolution (i.e. 0.25 mil). Pixels are dropped from the image and other pixels are duplicated as needed. These commanded shifts have the unfortunate effect, however, of distorting the image and introducing abrupt shifts of 0.25 mil in the photoplot. For example, if a shift were to occur exactly on a circuit trace, it would distort the width of this trace by the 0.25 mil error, which can be detrimental to the performance of the circuit.
It would be advantageous to have a system for providing compensation for inaccuracies in drum or flatbed raster photoplotters and scanners which would avoid abrupt shifts in the written substrate. The present invention is drawn toward such a system.